We will be including a freely redistributable POSIX regex library in a later release of OpenLDAP. Until then, on some systems an additional library or compiler flag may be required, such as -lposix. On a few (older) platforms, 3rd party libraries may be required. GNU Regex or Spencer's POSIX regex work fine. The "PORTS" document you refer to is for U-Mich LDAP and should be updated.